(1) Field of the Invention
This invention relates to an acrylonitrile polymer film and a process for the preparation thereof. More particularly, it relates to an acrylonitrile polymer film with highly reduced brittleness characteristics, namely, highly improved tensile elongation, folding endurance, and impact strength, and process for the preparation of this film.
(2) Description of the Prior Art
Films made of acrylonitrile polymers, especially polymers with a high nitrile content, have excellent gas barrier properties, weatherability, and stain resistance, but have inferior characteristics with regard to brittleness, such as tensile elongation, folding endurance, and impact strength. Moreover, since such polymers have poor thermoplasticity, the film should be prepared by a solvent shaping process using a solvent or a semi-melt shaping process using a plasticizer. However, the films obtained upon removing the solvent or plasticizer from the as-formed film have poor mechanical strengths, necessitating, orientation, and especially biaxial orientation.
Various methods for obtaining acrylonitrile polymer films having good mechanical strengths by biaxially stretching unoriented films of acrylonitrile polymer have already been proposed. However, when an acrylonitrile polymer film is oriented by stretching, fibrillation is readily caused, making biaxial stretching, and especially sequential biaxial stretching, difficult. As a typical instance of the methods which enable biaxial stretching of an acrylonitrile polymer film, there can be mentioned a method in which an unoriented film of an acrylonitrile polymer still containing a solvent, a plasticizer, or water is biaxially stretched. However, this method has drawbacks in connection with orientation and densification, making it impossible to obtain satisfactory improvements in the mechanical strength characteristics, and especially satisfactory reduction in the brittleness characteristics, of the film.
U.S. Pat. No. 3,437,717 discloses another method for biaxially stretching an unoriented acrylonitrile polymer film in the plasticized state, in which a solution of acrylonitrile polymer in a polar organic solvent is extruded in the form of a sheet and part of the solvent removed, following which the sheet biaxially stretched and the remainder of the solvent removed by drying with hot air. In this method, however, fluidization of molecules occurs, as a result of which sufficient orientation and densification are not attained.
U.S. Pat. No. 2,706,674 discloses a method in which an acrylonitrile polymer is placed in a semi-molten state by the hydration of the nitrile groups in the polymer at high temperature and pressure. This semi-molten acrylonitrile polymer is then extruded as a film through a slot die into an inert gas at room temperature. However, the film thus obtained has a large void ratio and an insufficiently high mechanical strength.
Japanese Examined Patent Publication No. 53-15,069 and U.S. Pat. No. 4,144,299 provides a method in which a solution of an acrylonitrile polymer is coagulated in an aqueous coagulating bath, and the film formed washed with water to remove the solvent, and the obtained undrawn water-containing film obtained biaxially stretched in the presence of water at a temperature of at least 100.degree. C. by a tenter method or an inflation method. The film thus obtained is highly oriented, but still has poor tensile elongation, and other brittleness characteristics.
In addition, U.S. Pat. No. 4,066,731 discloses a method in which an undrawn film of acrylonitrile polymer is biaxially stretched in the presence of water in a wet atmosphere at a temperature not higher than 90.degree. C. (200.degree. F.), and preferably at a temperature not higher than 70.degree. C. (160.degree. F.), following which it is densified by drying. Even by this method, however, sufficient densification is difficult to attain, and the brittleness characteristics are not sufficiently reduced.